Power supply and power conversion systems represent two major application areas for rectifying devices. Exemplary applications include AC-to-DC converters which use rectifier bridges, and boost and buck converters which use switches and rectifiers to deliver power from a source potential to a destination potential via an inductor storage device.
In the fields of power conversion and distribution, there are several ideal characteristics for a rectifier. To minimize power dissipation, the rectifier should conduct a high forward current with low associated forward voltage drop and conduct negligible reverse current for an applied reverse voltage. Second, the rectifier should have little variation of the rectification characteristics with respect to temperature for consistency of operation. Third, the rectifier should have a low reverse-recovery time and low reverse-recovery current to minimize current and voltage spikes. Fourth, for computer controlled power supply applications such as regulated boost and buck converters, the rectifier should provide some means for monitoring the amount of forward current to aid in the proper regulation of power and voltage.
Silicon-based diodes, specifically p-n junction diodes, are used for the majority of rectification applications. They, however, do not provide the most ideal rectification characteristics for power supply applications. First, silicon p-n junction diodes have a comparatively high forward voltage drop of 0.7 V. In contrast, field-effect transistors (e.g., MOSFET's, MESFET's) and bipolar-junction transistors (BJT's) have a conductive voltage drop on the order of 0.1 V to 0.2 V. Second, the rectification characteristics of p-n junction diodes are very sensitive to temperature in contrast to field-effect transistors. Third, the reverse recovery time of a diode and the amount of the reverse recovery current are large and increase as the diode is optimized for high forward-current conduction. In contrast, the generic FET structure can be optimized for high forward current conduction while maintaining or reducing its corresponding reverse-recovery characteristics.
Although they are typically used for current regulation applications rather than rectification applications, current-sensing devices address many of the above shortcomings of standard silicon p-n diodes. A current-sensing device generally comprises five terminals: first and second conduction terminals though which a main current flows, a modulation terminal for controlling the flow of the main current, and first and second sense terminals for providing a branch current which is proportional to the flow of the main current. A current-sensing device further comprises a plurality of main transistors coupled in parallel to one another and a sense transistor. The principle of the current-sensing device is to first conduct the main current through the main transistor and measure the voltage conditions present on the main transistors. The current-sensing device then replicates the measured voltage conditions onto the sense transistor and couples the current of the sense transistor to the first and second sense terminals. A small portion of the main current is conducted by the sense transistor.
A major drawback of using current-sensing devices in rectification applications is the need to generate a control signal for the modulation terminal. The control signal must be generated such that the main current flows efficiently in one direction through the current-sensing device. This condition requires that the control signal be synchronized to the applied voltage across the conduction terminals of the current-sensing device. For the majority of rectifier applications, the means for generating the control signal is both difficult and expensive. In the field of switching power supplies, however, the means for generating such a control signal is relatively simple and inexpensive. In many cases, such a signal is an intrinsic component of the switching power supply.
In switching power supply applications, however, there is a major difficulty in measuring the sense current of the current-sensing device. The standard passive techniques of measuring the sense current, e.g., a sense resistor, alters the applied voltages on the sense transistor and, hence, alters the value of the sense current. Active measurement techniques, such as those using differential amplifiers, do not significantly impact the applied voltages on the sense transistor and provide a more accurate measurement of the sense current. Differential amplifiers, however, require both positive and negative voltage supplies. Unfortunately, conversion and distribution applications often do not have a negative supply. The addition of the needed negative supply represents a large increase in cost, particularly for simple buck and boost converter topologies.
In summary, current-sensing devices could provide significant cost and performance benefits for switching power supply applications since they can be used to rectify current with relatively low power dissipation and can provide a measure of the rectifier current. The use of current-sensing devices is, however, hampered by the need for a negative supply.